Internal combustion engines derive power from a controlled combustion of a mixture of a hydro-carbon based fuel and air inside a combustion chamber. A primary goal of any engine design is to increase fuel efficiency and performance while reducing emissions. A more complete combustion reduces emissions such as unburned hydrocarbons, (referred to herein "THC" emissions) as well as carbon dioxide (CO.sub.2) and carbon monoxide (CO). Of course, while complete combustion is desired, design trade offs must be made since the processes leading to the most complete combustion may have negative side effects. For example, if the peak temperature of combustion is too high, undue NO.sub.X emissions will be formed during combustion and be exhausted. In addition, for a very hot flame front in the presence of cooler spots in the combustion chamber or compression end of the cylinder, so-called "knock" may occur, thus reducing engine performance. Of course, a myriad of other considerations go into the design of any engine.
One means of increasing fuel efficiency while reducing emissions is to provide a relatively fast burn of the combustion charge. The idea underlying such a design is that a faster burn will be more complete since the charge constituents (fuel and air) will preferably be close to the point at which combustion is initiated when they are burned. So-called fast burn is typically achieved by virtue of engine designs which seek to minimize the surface-to-volume ratio (S/V) of the combustion chamber. The smaller S/V thus promotes a fast burn. By "fast burn" it is meant that the combustion is such that most of the pressure exerted on the piston by combustion is exerted over a small portion of the piston's travel which occurs just following ignition of the charge. Thus, while the high heat of the fast burn is advantageous in that it gives a more complete burning of the hydrocarbons, it may have the disadvantage of leading to engine roughness and vibration by virtue of such a large force being exerted during a small portion of the piston travel. Moreover, such an engine must be designed such that the peak temperature of combustion is high enough to get the desired hydrocarbon burning, but not so high as to generate undue NO.sub.X.
Another approach to achieving more complete combustion for fuel efficiency and emissions purposes is to increase the homogeneity of the charge. A combustion charge may not be completely homogeneous, meaning that certain regions are more volatile than others leading to uneven combustion. In one engine, to prevent such problems, the fuel-air mixture was subjected to an induced swirling motion prior to ignition to increase the thoroughness of the mixture. As shown in U.S. Pat. No. 4,846,138 this swirling was induced by the valve stems extending across a thin, upstanding combustion chamber during the compression stroke of the flat-headed piston. The valve stems serve to induce a swirl in the fuel-air mixture being compressed leading to a more thorough mixing. Further, during the power stroke, the same configuration of the valve stems causes the flame front to swirl around these stems again improving the combustion. As discussed in that patent, the swirling induced by this configuration improved gas mileage and emissions for that engine.
The swirling during combustion in the engine according to the '138 patent was also beneficial in removing unburned hydrocarbons from the walls of the cylinder. Since the walls of the piston cylinder are typically cooler than the internal volume of the cylinder, unburned hydrocarbon molecules may cling to these walls during combustion. The swirling induced by the valve stems in the '138 patent help to sweep that charge around these cooler walls, thus assisting in removing unburned hydrocarbons. Such "scrubbing" of combustion chamber walls is thus a desirable feature for reducing such emissions.
While the extreme temperatures under which combustion typically occurs are advantageous in terms of burning the fuel efficiently, it has other draw backs which must be compensated for in the engine design. One example is in the valves associated with the combustion chamber. In a typical engine configuration, the intake and exhaust valves are disposed within the intake and exhaust ports which they are sealing. This is particularly disadvantageous in the case of the exhaust valve since that valve sits in the stream of hot exhaust gas as it leaves the combustion chamber and exits through the exhaust port to the exhaust manifold. Alternatively, the intake valve is almost continually subject to vacuum. Further, because of the configuration of conventional valves and their position relative to the cam shaft, typical valves are very long and have thin stems. Since flow to the stem away from the valve head is one means of heat transfer and dissipation, this means that a long thermal distance must be traversed to effectively draw heat away from the head in this manner. Further, the stems are typically very thin, meaning that only a small radiating surface is available for radiating heat in the stem. The other mechanism for cooling standard valves is flow from the head into the valve seat. Of course, this mechanism is unavailable when the valve is open and not in contact with the seat. Because typical valves are required to withstand incredibly high temperatures without having adequate mechanisms for withstanding such temperatures, they are typically formed of expensive and exotic materials so that they can successfully withstand the elevated temperatures. It would thus be desirable to avoid the undue expense and complexity of having to use such exotic materials for the valves.
A further consideration in regard to the valves is lubrication. A valve is generally actuated from a rotating cam shaft. In a typical single overhead cam, the cam contacts a cam pad at the end of a rocker arm. This contact causes the rocker arm to move the valve out of engagement with its respective port. The cam pads on the rocker arm, however, are typically located above the cam shaft, since the valves must be pulled out of engagement with their respective ports. As a result, lubricating this contact is problematic. Any oil that is thrown up to lubricate the cam/cam pad contact simply runs off due to gravity. A typical dual overhead cam arrangement suffers from similar problems. There, a cam engages the angled top surface of a bucket which houses the valve spring. Any oil thrown onto the top surface of the bucket also runs off due to gravity. While the cams are sufficiently lubricated to allow the engine to function, such lubrication is less than ideal and works against gravity, thus requiring an oversupply of oil to achieve lubrication.
The lubrication mechanism for the valve train itself is also less than ideal although it works for its intended purpose. Since the valves reciprocate in a bearing sleeve, there must be lubrication between the valve and the sleeve. This lubrication is typically carried out by a planned or intentional leakage of oil between the valve and the valve sleeve and past the valve seals. Thus, the valve seals are designed to have less than ideal sealing characteristics. In the case of the intake valve, the controlled leakage of oil in this manner is somewhat assisted by the fact that the intake port is constantly containing vacuum in the intake port. This assists in drawing lubricating oil between the bearing sleeve and the valve. Of course, this has the draw back of insuring that at least a small amount of oil is burned during each combustion cycle of a conventional engine. The exhaust valve, on the other hand, does not have such a mechanism for assisting in the movement of oil between the valve and the bearing sleeve. Because of the lack of such a mechanism this means that the exhaust valve and intake valve in a given engine typically see different amounts of oil, and are thus lubricated differently. Indeed, because of this, exhaust valves fail at a significantly higher rate than intake valves. In addition, the only way for oil in the bearing sleeve of an exhaust valve to exit is to be exhausted through the exhaust port during the exhaust strike of the piston with which a given valve is associated. This has negative impact in terms of emissions.
Lubrication of a typical valve train assembly is thus a difficulty which must be taken into account in engine design. Clearly, the system works, but extreme measures must be taken to compensate for the disadvantages of such systems.